We are studying experimentally the factors affecting peripheral venous capacity. We have recently developed a model describing the effects of internal temperature, mean skin temperature and exercise on forearm venous volume (FVV). This model should contribute to a more complete understanding of the mechanisms involved in heat syncope and other heat disorders, in which venous pooling plays a prominent etiological role. Forearm volume is measured with Whitney strain guages, and FVV is taken as the difference between the volume measured with the veins filled to a standard congesting pressure, and volume measured with the veins emptied by gravity. Studies of FVV will be supplemented with similar studies of calf venous volume on supine subjects, to test how well FVV reflects cutaneous venomotor activity elsewhere on the body. We will study the effect of local temperature by comparing bilateral measurements of FVV on forearms maintained at local temperatures which are different from each other. At several levels of venomotor activity, we will study venous compliance by varying the congesting pressure and measuring venous volume, and pressure within the forearm veins. We will investigate how the parameters of the basic model above are affected by a program of exercise training and heat acclimation. We will also investigate the role of the veins in core to skin heat transfer by determining the relationship between thermal conductance and skin blood flow at several different levels of venomotor tone. In addition, we will determine the effect of increased peripheral vascular pooling on stroke volume, which effect limits cardiac output under conditions of heat strain. Further work in this project includes investigating the effects of such factors as age, sex, and season on the control parameters, which will aid prediction of individual tolerance to heat stress. We will also investigate the control of ear venous volume in the rabbit, in order to develop an animal model on which to test new hypotheses in experiments not readily performed on humans.